A hydraulic valve system of an internal combustion engine can achieve continuous variation of maximum valve lift, valve opening duration angle and valve train phase at the same time, and such a valve system is called a Hydraulic Fully Variable Valve System (HFVVS). HFVVS has an important role in energy conservation and emission reduction for the internal combustion engine. Currently, the representative HFVVSs are the Schaeffler Uniair system, the Ford's Electronic-hydraulic Fully Variable Valve System, the Lotus's EHFVVT system and the like, which all use high-frequency solenoid valves as oil control switches to control hydraulic oil inflow and outflow. However, the high-frequency solenoid valves have the disadvantages of low frequency response speed, low reliability and high cost.
Invention patent No. ZL200610042070.9, entitled “Valve Train System of Internal Combustion Engine with Continuously Variable Valve Timing”, was published by Chinese State Intellectual Property Office in 2006. It provides a valve train system of an internal combustion engine with continuously variable valve timing, which includes a valve assembly, a hydraulic cylinder assembly, a hydraulic cylinder outlet control device, a hydraulic cylinder inlet control device, and a camshaft driving assembly. However, the system has the problem of instable working because the hydraulic low-pressure system has big pressure fluctuation in the process of oil discharging from the hydraulic cylinder. The hydraulic system has the problem of hydraulic oil leakage to result in air entry in the work stopping period of the internal combustion engine. The outlet control device of the hydraulic cylinder also has the limitation of being not applicable to a multi-cylinder internal combustion engine due to structure defects, and has the problem of unstable transmission. These problems lead to unfeasible popularization and application of this system.